The IPCC forecasts that ocean pH will fall by "between 0.14 and 0.35 units over the 21st Century, adding to the present decrease of 0.1 units since pre-industrial times"

"The much simpler forms support many fewer species, and theory suggests they create an environment that would be very vulnerable to other stresses."

In an even more acid part of the study site, with a pH of 7.7, the scientists report that "reef development ceased".

Here, seagrasses dominate the floor - but they lack the hard-shelled snails that normally live on their fronds.

This is the second published study of a "natural lab" for ocean acidification.

The first, from a site in Mediterranean, found snails with their shells disintegrating; but the PNG site offers a snapshot of the future that might be more applicable to the world's tropical coral hotspots.

"The results are complex, but their implications chilling," commented Alex Rogers from the University of Oxford, who was not part of the study team.

"Some may see this as a comforting study in that coral cover is maintained, but this is a false perception; the levels of seawater pH associated with a 4C warming completely change the face of reefs.

"We will see the collapse of many reefs long before the end of the century."

The scientific team behind the new research, drawn from Australia, Germany and the US, suggests that the picture from PNG may underplay the threat.

Reefs in the acidic zones of the study site receive regular doses of larvae floating in from nearby healthy corals, replenishing damaged stocks.

This would not be the case if low pH levels pertained throughout the oceans.

In addition, corals at the site are only minimally affected by other threats; there is little fishing, local pollution, or disease.

By contrast, a major survey published earlier this year found that three-quarters of the world's reefs were at risk - 95% in southeast Asia - with exploitative and destructive fishing being the biggest immediate threat.